Magnesium-based alloy

ABSTRACT

The invention relates to a magnesium-based alloy and a semi-finished product produced therefrom with uniformly small grain size and high cold forming capability. In order to create a fine-grain billet from an ingot, the material of which is highly formable (deep-drawable) at increased temperature and at room temperature and has desirable corrosion properties, according to the invention a magnesium-based alloy (L 1,  L 2 ) is provided, containing in % by weight zinc (Zn) more than 0.8, but less than 6.2, zirconium (Zr) traces, but less than 1.0, manganese (Mn) more than 0.04, but less than 0.6, calcium (Ca) more than 0.04, but less than 2.0, silicon (Si) traces, but less than 1.0, antimony (Sb) traces, but less than 0.5, silver (Ag) more than 0.1, but less than 2.0, the rest being magnesium and production-related contaminants.

The invention relates to a magnesium-based alloy and a semi-finished product produced therefrom.

To be specific, the invention relates to a magnesium-based alloy with uniformly small grain size and a high in particular cold forming capability of the material.

Magnesium is an alkaline earth metal, crystallized in hexagonally densest sphere packing of the atoms, has a density of 1.7 kg/dm³, a modulus of elasticity of 44 kN/mm² and a tensile strength of 150 to 200 N/mm². A hexagonally close-packed lattice has only a limited family of glide planes, so that magnesium is deformable at room temperature only to a limited extent.

Alkaline earth metals are generally very highly reactive. Magnesium is covered with a thin adherent oxidic/hydroxidic coat in air or water and is at least in part resistant with respect in particular to water. However, the high reactivity of magnesium, despite the protective surface layer, sometimes causes corrosion.

To increase the strength, reduce the notch sensitivity and improve the resistance to corrosion, magnesium can be alloyed primarily with the elements aluminum (Al), zinc (Zn), manganese (Mn), wherein in general these alloys are present at room temperature in a multiphase manner in the form of mixed crystals and intermetallic phases.

The toughness or ductility of the material comprising these alloys can be influenced by a solution annealing with subsequent quenching, and the strength of the material comprising these alloys can be influenced by slow cooling or precipitation hardening.

The designation and chemical composition of the most important currently customary magnesium alloys are listed in Table 1.

However, known magnesium alloys have the disadvantages of an inhomogeneous structure setting in the billet during extrusion at increased temperature and a limited ductility of the material at room temperature.

In particular the low density of the metal represents an important advantage of magnesium, so that for a long time experts have been confronted with the desire for magnesium-based forgeable alloys.

For example, it has become known from a publication, “The Effect of Ca Addition on Age Hardening Behaviors and Mechanical Properties in Mg-Zn Alloy” (Materials Science Forum Vols. 419-422 (2003) pp. 307-312) to add 0.1 to 0.5% by weight calcium to an alloy of magnesium and 6% by weight zinc in order to increase the mechanical properties and to improve the age-hardening parameters.

With the same objective of increasing strength and improving the creep resistance, according to the document “Microstructure and Mechanical Properties of Mg-Zn-Si-based alloys” (Materials Science and Engineering A357 (2003) 314-320) an alloying was carried out of 1% by weight Si and optionally 0.25% by weight Ca to a magnesium-based material with 6% by weight Zn.

In order to create high-strength and formed magnesium-based alloys, as disclosed in the document “Microstructure and Mechanical Properties of Mg-Zn-Ag Alloys” (Materials Science Forum Vols. 419-422 (2003) pp. 159-164), an attempt was also made to add silver (Ag) to an alloy Z6, wherein it was possible to achieve a remarkable grain refinement and an increase in hardness with a content of 3% by weight Ag.

The object of the invention is now to create a magnesium-based alloy that yields a fine-grain extrusion billet during a hot pressing of an optionally conditioned continuous casting billet, wherein the material of the same is highly formable at increased temperature and at room temperature. Furthermore, an object of the invention is to improve or to influence the corrosion resistance of the material.

This object is attained with a magnesium-based alloy containing in % by weight

Zinc (Zn) more than 0.8, but less than 6.2 Zirconium (Zr) traces, but less than 1.0 Manganese (Mn) more than 0.04, but less than 0.6 Calcium (Ca) more than 0.04, but less than 2.0 Silicon (Si) traces, but less than 1.0 Antimony (Sb) traces, but less than 0.5 Aluminum (Al) traces, but less than 0.5 Silver (Ag) more than 0.1, but less than 2.0 the rest being magnesium and production-related impurities.

The advantages achieved with the magnesium-based alloy composed according to the invention lie essentially in a strictly balanced concentration of elements and a microalloy technology in which the interaction of all of the alloying elements and the chemical kinetics and the crystalline growth criteria are taken into consideration, wherein the advantages represent in particular a homogeneous fine grain structure of the material, a high cold workability and an improvement in the corrosion resistance.

Zinc in contents of more than 0.8 to less than 6.2% by weight in the alloy influences the solidification range decisively and prevents a formation of very coarse columnar crystals during solidification. Lower concentrations than 0.8% by weight Zn lead to a disproportionately decreasing effect, but contents of more than 6.2% by weight produce a disadvantageously acting eutectic solidification of the melt.

Zirconium has a grain refining effect through precipitations from the melt and in enrichment on the crystallization front. Contents of over 1.0% by weight Zr coarsen the precipitations in a disadvantageous manner for the crack-initiation of the material under loads.

Manganese in contents of more than 0.04, but less than 0.6% by weight has multiple effects in the alloy. On the one hand, Mn sets Fe in the melt, which compound precipitates, on the other hand, Mn with zirconium already forms phases in the melt at higher temperature that can have a grain refining effect.

Calcium with contents of more than 0.04, but less than 2.0% by weight in the metal produces a phase formation in the solid alloy, which phases as grain-boundary stabilizer effectively prevent a crystal growth. This Ca₂Mg₆Zn₃ phase, which presupposes Zn in the alloy in the contents referenced above, forms in the range of 0.1 to 1% by vol. distributed particularly finely and homogeneously in the material, whereby an excellent fine grain structure is maintained in the material.

In conventional magnesium alloys, as a rule grain boundary-stabilizing precipitations are electrochemically more noble than the magnesium matrix, so that the corrosion resistance is impaired by galvanic effects. In the alloy according to the invention, the base Ca₂Mg₆Zn₃ phase precipitates, so that a galvanic corrosion mechanism is significantly reduced. The result is an improved corrosion resistance.

The alloying element silicon is soluble in magnesium only to a very slight extent or in traces and forms the phase Mg₂Si. Above 1.0% by weight Si the phase proportion in the material of the alloy is large and impairs the mechanical properties thereof.

Antimony is essentially to be seen in connection with silicon, because antimony can produce a modification of the Mg₂Si phase, wherein a necessary Sb concentration in the alloying metal should be approx. half of that of the Si.

Although magnesium-based alloys, which can contain aluminum up to 8% by weight and more, definitely also have an application potential with respect to an increased material strength and creep resistance, aluminum represents an undesirable element in the material according to the invention. Through contents of greater than 0.5% by weight, brittle grain boundary phases of the Mg₁₇Al₁₂ type can form, which also have a corrosion-promoting effect in coarse embodiment. Furthermore, cracks form during the extrusion of the material under approx. 230° C., which cracks can lead to a brittle pressed article, wherein it can also have considerable grain size differences over the cross section and the longitudinal direction.

As a crystalline growth-inhibiting element, silver has a high potential in the alloy according to the invention in contents of more than 0. 1, but less than 2.0% by weight. In these concentrations, Ag is in solution in the hot state of the alloyed material, wherein, as was found, at contents of over 0.1% by weight Ag a concentration increase is formed at the grain boundaries, which highly effectively counteracts a crystalline growth. Furthermore, through Ag a curing effect of the material can be achieved via the Mg₄Ag phase. Higher Ag contents than 2.0% by weight have in particular economic and corrosion chemical disadvantages.

Preferred chemical compositions of the magnesium-based alloys according to the invention are given in Claims 2 and 3.

The total concentration of the microalloying elements Mn, Ca and Si of greater than 0.1 but less than 0.65% by weight in the magnesium-based material is of particular importance, as was found, for a homogeneous fine-grain fine structure and a high formability of an object of the alloy according to the invention in the range of room temperature.

A semi-finished product of a magnesium-based alloy according to the invention, which with a cross section/area ratio of greater than 1:16, in particular greater than 1:20 was deformed from a cast billet to a pressed article at a temperature of approx. 380° C., has a grain size of the structure of less than 10 μm, namely with extensive isotropy based on the cross section and in the longitudinal direction. Pressed articles according to the invention can be further deformed or pressed at temperatures below 200° C., in particular at room temperature, wherein a faultless surface or gloss surface can be achieved.

The invention is substantiated below based on several test results.

Table 2 shows the chemical composition of the materials tested.

The figures show:

FIG. 1 Stress-strain behavior of tested alloys

FIG. 2 Test alloy L1, cast structure

FIG. 3 x Test alloy L1, cast structure

FIG. 3.1 Magnification scale: 500 μm

FIG. 3.2 Magnification scale: 200 μm

FIG. 3.3 Magnification scale: 50 μm

FIG. 3.4 Magnification scale: 20 μm

FIG. 4 x Test alloy L1, deformed

FIG. 4.1 Traverse section edge

FIG. 4.2 Traverse section center

FIG. 4.3 Longitudinal section edge

FIG. 4.4 Longitudinal section center

FIG. 5 Test alloy L2, cast structure

FIG. 6 x Test alloy L2, cast structure

FIG. 6.1 Magnification scale: 500 μm

FIG. 6.2 Magnification scale: 200 μm

FIG. 6.3 Magnification scale: 50 μm

FIG. 6.4 Magnification scale: 20 μm

FIG. 7 x Test alloy L2

FIG. 7.1 Traverse section edge

FIG. 7.2 Traverse section center

FIG. 7.3 Longitudinal section edge

FIG. 7.4 Longitudinal section center

FIG. 8 Comparison alloy AZ31, cast state

FIG. 9 Comparison alloy ZK31, cast state

FIG. 1 shows the result of the expansion as a function of the stress in the tensile strength test according to EN 10002-1:2001 of magnesium-based alloys.

The alloy designations and alloy compositions given in Table 2 are referred to below.

The sample with designation L1 of an alloy according to the invention with a deformation by means of indirect press molding and with a press ratio of 1:25 yielded in the tensile strength test (A50) at room temperature an expansion of over 25% at a maximum stress of approx. 260 MPa.

A yield strength of Rp_(0.2)=330 MPa of the material was determined on the sample from another test alloy L2 according to the invention after an identical press molding of the ingot at 380° C. at room temperature, wherein an expansion value of greater than 15%, in this case of approx. 19%, was available as a gauge of ductility.

As shown by FIG. 1, the comparison alloys ZK31, AZ31 and ZM21 exhibited throughout lower breaking elongation values Ac than the materials according to the invention.

The dendritic cast structure of the alloy L1 can be seen from FIG. 2. An average grain size of 140 μm was determined with essentially homogeneous structure over the entire cross section of the ingot.

FIG. 3 shows the most homogeneous structure in the cast state of the ingot of alloy L1 over the cross section in various magnifications with a scale of 500 μm (FIG. 3.1), 200 μm

(FIG. 3.2), 50 μm (FIG. 3.3) and 20 μm (FIG. 3.4) and shows spherical grains with some grain boundary phases.

FIG. 4 shows a material of the alloy according to the invention L1 deformed with a press ratio of 1:25 at 380° C. in longitudinal and transverse direction from the edge and central area of the sample.

FIG. 4.1 and FIG. 4.2 are transverse section images from the edge and the center of the rod, wherein FIG. 4.3 and FIG. 4.4 represent the corresponding longitudinal section images. An average grain size of 9 μm to 6 μm was measured.

FIG. 5 shows the globulitic cast structure of an alloy L2 according to the invention. With extremely homogeneous grain distribution over the ingot, the average grain size was 40 μm.

FIG. 6 shows the cast structure of FIG. 5 (L2) in its very fine embodiment with scales of 500 μm (FIG. 6.1), 200 μm (FIG. 6.2), 50 μm (FIG. 6.3) and 20 μm (FIG. 6.4). Small fine precipitation phases can be detected at the grain boundaries.

FIG. 7 shows the structure of a pressed article of an alloy L2 of an ingot pressed at a temperature of 380° C. with a press ratio of 1:25 in the transverse direction at the edge (FIG. 7.1) and in the central area (FIG. 7.2) as well as in the longitudinal direction at the edge (FIG. 7.3) and in the central area of the rod (FIG. 7.4). The average grain size was approx. 2 μm.

FIG. 8 shows the cast structure of an ingot of a comparison alloy AZ31. A measurement of the microstructure produced a grain size of 360 μm with essentially homogeneous distribution over the cross section.

After an extrusion at 380° C., the structure was in part coarsely recrystallized and inhomogeneous, whereby no reliable determination of the grain size was possible.

As shown in FIG. 9, the cast structure (ingot casting) in the ingot of the comparison alloy ZK31 was globulitic and exhibited a grain size of 80 μm with good homogeneity over the cross section.

After a hot pressing of the ingot, the extrusion profile was partially inhomogeneously recrystallized. A determination of the crystal size with a certain degree of certainty was not possible on the extruded profile.

TABLE 1 Compositions of magnesium alloys according to the prior art (in % by weight) Alloy designation Zn Mn Al Si Ca Zr Mg Z6 6.0 Rest ZM 21 2.0 1.0 Rest ZK 31 3.0 0.6 Rest AZ 91* 0.8 0.4 9.0 Max. 0.5 Rest AM 60* 0.25 6.0 Rest AZ 31 1.0 Up to 1.0 3.0 Rest *essentially cast alloys

TABLE 2 Compositions of tested materials (in % by weight) Alloy designation Zn Mn Al Si Ca Zr Ag Mg L1 2.9 0.2 — 0.2 0.5 Rest L2 2.8 0.1 — 0.2 0.8 0.4 Rest ZM 21** 1.8 0.7 Rest ZK 31** 2.7 0.5 Rest AZ 31 1.0 0.3 3.0 Rest **no standard alloys 

1. Fine-grain magnesium-based alloy containing in % by weight Zinc (Zn) More than 0.8, but less than 6.2 Zirconium (Zr) traces, but less than 1.0 Manganese (Mn) More than 0.04, but less than 0.6 Calcium (Ca) More than 0.04, but less than 2.0 Silicon (Si) traces, but less than 1.0 Antimony (Sb) traces, but less than 0.5 Silver (Ag) More than 0.1, but less than 2.0

the rest being magnesium and production-related impurities.
 2. Magnesium-based alloy according to claim 1 in which the microalloying elements Mn, Ca and Si have a total concentration of greater than 0.1, but less than 0.65, preferably of greater than 0.15, but less than 0.5.
 3. Magnesium-based alloy according to claim 1, in which the concentration in % by weight of one or more alloying elements is Zn more than 1.0, preferably more than 1.5, but less than 5.9, preferably less than 4.0 Zr less than 0.8, preferably less than 0.6 Mn more than 0.06, preferably more than 0.09 but less than 0.4, preferably less than 0.2 Ca more than
 0. 1, preferably more than 0.14 but less than 1.0, preferably less than 0.6 Si less than 0.5, preferably less than 0.2 Sb less than 0.25, preferably less than 0.1 Al less than 0.1, preferably less than 0.08 Ag more than 0.2, preferably more than 0.38 but less than 1.2, preferably less than 0.9
 4. Semi-finished product of a magnesium-based alloy with a chemical composition according to claim 1, deformed with a press ratio of at least 1:20, which semi-finished product has a grain size of less than 10 μm and has extensive isotropy. 